NAME

SYNOPSIS

DESCRIPTION

unshare()
allows a process to disassociate parts of its execution
context that are currently being shared with other processes.
Part of the execution context, such as the mount namespace, is shared
implicitly when a new process is created using
fork(2)
or
vfork(2),
while other parts, such as virtual memory, may be
shared by explicit request when creating a process using
clone(2).

The main use of
unshare()
is to allow a process to control its
shared execution context without creating a new process.

The
flags
argument is a bit mask that specifies which parts of
the execution context should be unshared.
This argument is specified by ORing together zero or more
of the following constants:

CLONE_FILES

Reverse the effect of the
clone(2)
CLONE_FILES
flag.
Unshare the file descriptor table, so that the calling process
no longer shares its file descriptors with any other process.

CLONE_FS

Reverse the effect of the
clone(2)
CLONE_FS
flag.
Unshare file system attributes, so that the calling process
no longer shares its root directory, current directory,
or umask attributes with any other process.
chroot(2),
chdir(2),
or
umask(2)

CLONE_NEWNS

This flag has the
same
effect as the
clone(2)
CLONE_NEWNS
flag.
Unshare the mount namespace,
so that the calling process has a private copy of
its namespace which is not shared with any other process.
Specifying this flag automatically implies
CLONE_FS
as well.

If
flags
is specified as zero, then
unshare()
is a no-op;
no changes are made to the calling process's execution context.

RETURN VALUE

On success, zero returned.
On failure, -1 is returned and
errno
is set to indicate the error.

ERRORS

EINVAL

An invalid bit was specified in
flags.

ENOMEM

Cannot allocate sufficient memory to copy parts of caller's
context that need to be unshared.

EPERM

flags
specified
CLONE_NEWNS
but the calling process was not privileged (did not have the
CAP_SYS_ADMIN
capability).

VERSIONS

The
unshare()
system call was added to Linux in kernel 2.6.16.

CONFORMING TO

The
unshare()
system call is Linux-specific.

NOTES

Not all of the process attributes that can be shared when
a new process is created using
clone(2)
can be unshared using
unshare().
In particular, as at kernel 2.6.16,
unshare()
does not implement flags that reverse the effects of
CLONE_SIGHAND,
CLONE_SYSVSEM,
CLONE_THREAD,
or
CLONE_VM.
Such functionality may be added in the future, if required.